Deleading zinc-lead fume



Patented June 1, 1948 UNITED STATES DELEADING ZINC-LEAD FUME Iv Carlos Bardwell, Tooele, Utah, assignor to International smelting & Refining Company, New York, N. Y., a corporation of Montana No Drawing. Application September 9, 1943,

Serial No. 501,671

8 Claims. (Cl. 23-148) This invention relates broadly to the removal of lead from metallurgical products containing zinc and lead principally in the form of oxides in which the lead is an objectionable impurity. More particularly, the invention relates to the re- 5 moval of lead from zinc oxide fume products containing several per cent of lead. The invention aims to provide certain improvements in the removal of lead from such products.

The invention is particularly applicable to the deleading of the zinc oxide fume which is obtained by blowing lead blast, furnace slag with air and powdered coal in a slag treating furnace. The fume of this operation is composed largely of zinc oxide and contains several percent of lead oxide, a typical fume having an average analysis of 65% Zn and 9% Pb. This fume has heretofore been deleaded by passing it through a brick-lined rotary kiln, 72 feet long and 7 feet in diameter, mounted at a slight inclination and heated by combustion gas applied at the discharge end of the kiln to a temperature as high as 2400" F. in the hottest zone. The fume takes about 45 to 55 minutes to pass from the upper or feed end of the kiln to the lower or discharge end, during which period it is turned over and over by the rotationof the kiln (one revolution in about one minute), and is continuously submittedto temperatures varying from 1000 to 1200 F. at the feed end to about 2350 to 2400 F. near the discharge end, depending upon the type of operation. In this treatment of the fume, most of the lead oxide is volatilized without the volatilization of much zinc oxide, and the volatilized lead oxide together with the hot gases from the kiln pass through long, cooling fines and then through a baghouse where the lead oxide fume is caught. This so-called lead fume has an analysis of about 50% Pb and 10% Zn. The high maximum temperature (2350 F. or higher) required in this operation causes rapid deterioration of the brick lining in the kiln, thus frequently interrupting the operation and increasing the cost because of the frequent repairs to the lining. Moreover, when treating capacity tonnages, only fair lead 5 elimination is obtained. More recently, it has been found that the addition to the fume of a small amount of crude sodium chloride (common salt) is beneficial in the aforementioned rotary kiln operation. Thus, about 2% of salt mixed with the fume aids the removal of lead by its chemical, and perhaps catalytic, action. The benefits resulting from this use of salt are (1) better deleading of larger tonnages of fume, and (2) lower repair costs and more con- 2 tinuous operation due to lower maximum kiln temperatures (about 2250 to 2300 F. with salt instead of 2350 F. when treating fume alone). However, the use of salt gives rise to two serious objections, namely (1) the destructive corrosive effect of chlorine in the resulting lead fume on lines and baghouse collecting bags (wool), and (2) very undesirable, accumulative effects in the lead plant where the lead chloride fume is subsequently treated.

The present invention particularly contemplates an improvement upon these heretofore known practices of deleading crude zinc oxide fume. I have discovered that the addition of a small amount of. coke to the fume promotes volatilization of the lead contentof the product, and under properly controlled conditions makes possible a selective removal from the fume of a large proportion of the lead. Based on this discovery,

the present invention involves mixing the fume, or other similar metallurgical product containing oxides of zinc and lead, with a carefully controlled amount of coke, and passing the mixture through a heating zone having a temperature gradient from the entrance to the discharge points of about 1000" F. .to 2000 F. The rotary kiln hereinbefore mentioned is admirably adapted to provide the heating zone-and to this-end may :be heated by the combustion of natural gas-or equivalent fuel applied to the discharge end of the kiln-.- The maximum kiln temperature-(near the discharge end) is-at least 2000? F. and preferably about 2300 'F. and the temperature of the feed end of the kiln is from 1000 to 1200 F. Thus, from the feed end to the discharge end of the kiln the temperature progressively increases, and lead oxide is selectively reduced by the coke before the charge reaches a temperature sufiiciently high to reduce zinc oxide. The coke in the charge is thus utilized in large part in reducing lead oxide, and by the time the charge reaches a temperature sufliciently high to reduce zinc oxide the unconsumed coke is insufficient to reduce an object'ionable or uneconomic amount of zinc oxide.

The amount ofcoke admixed with .the'fume (or the like) is more or less critical. It should be in excess of the amount required to reduce all of the lead oxide to metalliclead but very substantially less than required to reduce the zinc oxide to metallic zinc. Since the fume or similar product contains several times as muchzincas lead, .an amount of coke aboutdouble that-required to reduce all of the lead oxide to metallic lead is roughly speaking, satisfactory. Thus, the

amount of coke admixed with the fume may be taining from about 8 to about 20% of lead, from I 1 to 4% by weight of coke ordinarily give good results in the practice of the invention.

Fume products are inherently finely divided, in

fact powdery, and need no preliminary condi' tioning. may perhaps need to be ground or comminut'ed, since the materials constituting thecharge to the Other similar metallurgical products be in a finely divided state. The coke should be ground, coke breeze of minus 4 inch size being well adapted for the purpose.

While a rotary kiln is a particularly advantage'ous type of apparatus for providingthe characteristic temperature gradient or the invention, any other appropriate means for heating-the-mixture of fume (or similar product) and controlle ad amount of coke to a temperature progressively increasing from about 1000 F, to at least 2000 F. may be utilized. The temperature gradient is correlated to the time of passage of the charge therethrough to effect the reduction of substantially all of the lead oxide before the charge is heated-to a temperature at which zinc oxide is easily reduced by coke. Generally speaking, a time interval bf from 30 to 60 minutes for the passage of the charge through the temperature gradient is satisfactory.

In practicing the invention in an inclined brick-lined rotary kiln so operated that the charge passes therethrough in 45 to 55 minutes, the mixture of fume and coke enters the feed end of the kiln cold, and is gradually heated as it advances through the kiln to temperatures progressively increasing from about 1000 C. to at least 2000 F. Lead oxide is readily reduced to metallic lead by coke at temperatures startin at about 1000 F., while zinc; oxide is less easily reduced by coke at temperatures starting at about 1800-1850 F. Thus, the lead oxide is, reduced by the coke as the charge progresses through the zone of lower temperatures and beifor'e a su'iiiciently high temperature is reached to reduce zinc oxide. Because of the limited amount 'of coke used, most of this reducing agent (coke) is consumed in the reduction of lead oxide to lead and, very little coke remains for subsequent, reduction of'zinc oxide when the charge has progresed through the kiln to the zone of higher temperatures. Thus, there is a marked selective reduction of lead oxide. The atmosphere within the kiln is approximately neutral, theamount of air introduced being only such as to furnish sumcient oxygen for combustion of the heating fuel and to leave about 1 to 2% of excess oxygen. in the gases passing from the kiln. In this atmosphere, however, the lead again is promptly oxidized to lead oxide and is withdrawn from the kiln along with the gaseous products of the operation. The resulting lead fume is collected in a bag-house in the heretofore customary manner.

Precautions should be taken in feeding arotary kiln to prevent dusting. The mixture of fume and cokemay advantageously be fed to the kiln through a water-cooled screw conveyor so that the charge has the lowest possible drop from the bottom of the conveyor onto the lining of the kiln. The flue draft at the gas outlet of the kiln is held as low as possible in order to prevent excessive dusting,

In an actual practice of the invention for deleading a zinc-lead fume obtained from blowing a lead blast furnace slag with air and powdered coal, carefully measured amounts of coke breeze were thoroughly mixed with the fume by throwing the coke over the surface of the fume in the bag-h'ousepits, after which the mixture was carried by pneumatic conveyor to the feed hopper at the kiln. The fume analyzed 65% zinc and 9% lead. ial pounds or coke breeze (minus 4" size) were mixed'wit'h every pounds of fume. The

kiln was72 feetlon and 7 feet in diameter, and "made one revolution per minute.

The kiln was fired by natural gas to a maximum temperature of 2300" F. near the discharge end, and the temperature at the feed end was about 1000 F. The deleaded zinc fume produced in this operation contained 0.5 to 1% lead and about 77% zinc, The lead fume produced in the operation contaihed about 50% lead and 10% zinc.

The cost of coke used in the practice of the invention is considerably less than the cost of common salt used in the heretofore described procedure of the prior art. Moreover, the invention enables the production of a considerably higher grade of deleaded zinc fume than can be produced with salt (77% Zn with coke compared to 74% Zn with salt). The kiln temperatures necessary in the practice of the invention are not unduly detrimental to the brick lining, and the coke causes no corrosive action of the lining and imparts he undesirable properties to the lead fume.

I claim:

1'. The improvement in removing lead from a metallurgical product containing zinc and lead principally in the form of oxides with the amount of zinc several times the amount of-lead, which comprises mixing said product with coke in amount about double that required to reduce all of the lead oxide in the product to metallic lead. and heating the resulting mixture at a temperature progressively increasing from about 1000 F. to at least 2000 'F. by burnin heating fuel in contact with it, in the course of which lead oxide content of the product is reduced to metallic lead which vol'atilizes and separates from the zinc oxide, said heating being conducted in an atmosphere containing only a very small amount of oxygen in excess of that necessary to efiect substantially complete combustion of the heating fuel, said excess of oxygen being sufiicient to reoxidize the volatilized lead, and removing said reoxidized lead from the heating zone along with the products of combustion discharged therefrom,

2. Theimprovement in removing lead from a metallurgical product containing zinc and lead principally in the form of oxides with the amount of zinc several times the amount of the lead; which comprises mixing saidproduct with coke in amount about double that required to reduce all of the lead oxide in the product to metallic lead, and heating the resulting mixture at a temperature progressively increasing from about 1000 F. to at least 2000 in the course of which the lead oxide content of the product is reduced to metallic lead which volatilizes and separates 'from the zinc oxide, said heating tak-' ing place while said mixture is being contacted with a flowing gaseous stream containing only a slight amount of free oxygen, but suiiicient to reoxidize the volati-lized lead, and removing said:

reoxidized lead from the heating zone while sus-v pended in said gaseous stream.

3. The improvement in removing lead from a metallurgical product as set forth in claim 1 in which the metallurgical product is zinc oxide fume containing several percent of lead, and the heating takes place in a rotary kiln.

4. The improvement in removing lead from a. metallurgical product as set forth in claim 2 in; which the metallurgical product is zinc oxide fume containing several percent of lead, and the amount of coke is very substantially less than that required to reduce the zinc oxide.

5. The improvement in removing lead from a metallurgical product containing zinc and lead principally in the form of oxides with the amount of zinc several times the amount of lead, which comprises mixing said product with coke in amount from about 0.1 to 0.2 pound for each percent of lead in one hundred pounds of the metallurgical product, and heating the resulting mixture to a temperature progressively increasing from about 1000 F. to at least 2000 F. by burning heating fuel in contact with it, in the course of which lead oxide content of the product is reduced to metallic lead which volatilizes and separates from the zinc oxide, said heating being conducted in an atmosphere containing only a very small amount of oxygen in excess of that necessary to effect substantially complete combustion of the heating fuel, said excess of oxygen being sufiicient to reoxidize the volatilized lead, and removing said reoxidized lead from the heating zone along with the products of combustion discharged therefrom.

6. The improvement in removing lead from a metallurgical product as set forth in claim 5 in which the amount of coke is about 0.12 pound for each percent of lead in one hundred pounds of the metallurgical product.

7. The improvement in removing lead from a zinc oxide fume product containing at least 8% of lead in the form of lead oxide and several times as much zinc in the form of oxide,

which comprises mixing said fume with coke breeze in amount from about 1% to 4% by weighton the weight of the fume product, passing the.

resulting mixture through a rotating kiln heated by gaseous products of combustion to a tem-= perature of about 2300 F. at the discharge end and about 1000 F. at the feed end, in the course of which lead oxide content of the product is reduced to metallic lead which volatilizes andv separates from the zinc oxide, said heating taking place while said mixture is being contacted with a flowing gaseous stream containing only a:

very small amount of oxygen, but suificient oxy-'.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 959,924 Dedolph May 31, 1910 1,628,952 Cregan May 17, 1927 FOREIGN PATENTS Number Country Date 17,588 Sweden Apr. 10, 1902 265,558 Great Britain June 9, 1927 Certificate of Correction Patent No. 2,442,674. June 1, 1948.

CARLOS BARDWELL It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Column 1, line 30, for 2400 F read 2400 F.; column 3, line 42, for 1000 0. read 1000 F.; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 20th day of July, A. D. 1948.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

